3.6L GM Misfire - Why No Check Engine Light
How do you fix a misfire problem that's not being recognized by the PCM as a problem?! As the 4th shop to now take a look at this 2014 Chevy Impala 3.6L for a phantom misfire problem that seems to happen at light load and deceleration situations, we weren't going to let this customer down. This poor customer had already bought new spark plugs, 6 high pressure fuel injectors, multiple valve cleaning jobs, intake manifold gaskets, coils swapped, a throttle body and likely more all to chase down a problem that never set a check engine light. So, like we do with every diagnosis, we put this problem vehicle through our diagnostic process.
Starting with pulling trouble codes, as you may have guessed, none were found. From there a quick under hood visual inspection revealed many new parts but nothing out of the ordinary that led us to the fix. Up next was a test drive and review of the engine data. Our test drive revealed a slight misfire condition that could just barely be felt, but it was there. Looking at the scan tool showed us that the PCM was seeing a misfire on cylinders 1, 3 and 5; but it wasn't enough to set the check engine light on. Now knowing that the misfire problem appears to be only on bank 1, it was time to look at the fuel trims. Reviewing both long and short terms trims for both banks helped to pin this as a single bank problem. Bank 1, containing cylinders 1, 3 and 5, showed a -14% long term fuel trim while bank 2 show long term trims at nearly 0. Now -14% isn't a huge number, but it is outside of the "normal" +/- 10% we want to see. So while not coding for a rich condition, the PCM is definitely subtracting fuel from bank 1.
With the problem seemingly only affecting one bank of the engine, we went after one of the simplest tests to confirm the engine is mechanically sound; relative compression using a labscope. Hooking up our amp clamp around the battery cable and a test lead to the ignition coil on cylinder 2 for a trigger, it was time to disable fuel and crank the engine over for a look close look at the compression spikes. Our relative compression test results were questionable. The test was run successfully but the results weren't conclusive enough to condemn this misfire problem as an engine mechanical issue.
Hooking up a pressure transducer to the intake manifold for a running vacuum waveform was the next on the list because our relative compression test was inconclusive. This gave as a better look at how the engine was breathing. Finally we saw something that was conclusive. Every other vacuum peak was higher and every other valley was lower. Applying the GM 3.6L firing order, 1-2-3-4-5-6, told us that every other vacuum pull being consistent again pointed to one bank, or half of the engine. A breathing issue shown on the scope compounded with a rich condition and a subtle misfire on one bank led to the next test.
Now, if you know the GM 3.6L you could probably have guessed that we would end up at this point eventually. This 3.6L engine runs 3 timing chains, a main chain between the crankshaft and 2 drive sprockets that drive each banks timing chain. Because the issue was related to a single bank, it isn't unreasonable to assume a problem with the engine mechanical timing on bank one. So, we hooked the labscope up to the 2 cam sensors for bank 1 and the crank sensor and performed a cam/crank correlation test. Looking at our captured waveform vs a known good immediately confirms our suspicions. The cross point of the intake camshaft and the crankshaft signal on our vehicle occurred just before 4 teeth on our vehicle and the known good shows it is occurring just before 5 teeth, a full tooth difference. As with all scope captures, because our cross point of cam/crank was shifted to the right it was happening later in time. Simply put, the bank 1 timing was slightly retarded in time. Chain stretch is the most common cause of this concern on the 3.6L. Looking at the cross point of the exhaust camshaft sensor and the crank sensor displayed a similar story with our vehicle crossing at 4 teeth and the known good at 3 teeth. A quick look at the same test on bank 2 reveled timing that was identical to the known good.
So, to summarize, a subtle non-check engine light setting misfire that had the parts cannon fired at it appears to be caused by a stretched bank 1 timing chain. Unfortunately we weren't able to see this entire repair through as the car was returned back to the shop that called us but we are confident in our diagnosis based upon the facts; bank 1 misfires, bank 1 fuel trim issues and bank 1 timing issues. In the end, it's just another GM 3.6L V6 that needs timing chains, but for us, it was a fun way to get there without a cam/crank correlation style trouble code.
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